Operation of cable lines under fire conditions

. Branched cable communications are carriers of fire load, they are tracks along which fire spreads in buildings and structures. Therefore, currently, fire safety of cables is one of the most important tasks of the cable industry. The spread of flames in vertical bunch wiring depends on the following factors: the amount of combustible material that is exposed to high temperatures, geometric shapes of cables and their mutual location when laying, the ignition temperature of gases emitted by cables, the volume of combustible gases emitted by cables at a certain elevated temperature, the room capacity and volume of air passing through the cable structure, cable devices. Analysis of regulatory documents revealed some key differences in the methodology of standards. It was found that the spread of fire is not only affected by the volume of laid cables, but also by their mutual placement in space. A direct correlation was found between the influence of the cross-section of the cable core and preservation of the cable line in case of fire. Parameters were determined for the preservation of serviceability under fire exposure in the case of bunch and in the case of single wiring. It has been determined that it is impossible to spread the results of fire tests in case of single wiring in accordance with GOST IEC 60331-21-2011 and in case of bunch wiring in accordance with GOST IEC 60332-3-22-2011 to the results of tests in accordance with GOST R 53316-2009 for the preservation of operability of cable lines. The parameters of fire resistance and operability of cable products under the influence of flame depending on the type of cables and the method of their installation have been determined.


Introduction
Fire safety of cable lines must be considered as a complex set of different factors. Cables have a very complex multi-component design that combines combustible materials (electrical insulation, cable sheathing, etc.) and internal heat sources -electrically heated conductive cores, which in emergency modes of operation can become sources of fire and provoke fire development [1][2][3].
Analysis of statistical data shows that the causes of 20-25% of fires annually occurring in Russia are related to the operation of electrical installations. The same ratio is typical for It has been established that burning polymer compositions produce suffocating and poisonous substances such as carbon monoxide, nitrogen oxide, hydrogen sulfide, hydrogen chloride, formaldehyde and several other compounds, which, if inhaled, may provoke respiratory tract disorders or cause death [18][19][20]. Particularly dangerous is carbon oxide, which is formed by burning almost all materials and in most cases is the cause of casualties in fires.
When polymeric insulation compositions and cable sheaths are destroyed and burned, gaseous substances such as chlorine, bromine, fluorine, sulfur dioxide and others are released, which, when combined with water vapors, form acids or alkalis are capable of causing corrosion of metal structures and corrosion damage to electronic equipment. Although the corrosive dynamics of combustion products itself does not have a significant impact on the formation of fire, one should take this factor into account when designing cables, since the corrosive destruction of steel structures and equipment generates collateral damage from fire, which is significantly higher than the price of burned cables.
A general requirement for cables intended for laying in cable structures is nonproliferation of combustion. This is one of the most important characteristics of the cable, indicating the ability of the cable to stop burning independently after removing the ignition source. At the same time, there are requirements for non-proliferation of combustion for a single cable sample and requirements for nonproliferation of combustion for bunch wiring.
Cables with increased fire safety are cable systems that include trays-crops, perforated and non-perforated, wire trays, wall and ceiling hangers, cantilever brackets, mounting profiles, studs, anchors, fasteners, as well as many auxiliary devices to all types of products with a certain fire resistance limit.
For fire-resistant cables either single or bunched wiring is used (several units in a line) [6]. The selection of the method of laying is determined by the type of fire-resistant cable lines themselves, as well as the features of the project.
The use of cables that meet the requirement for non-proliferation of combustion for a single sample, in bunch wiring may lead to the spread of flame through the cables. This is because flame propagation in vertical bunch wiring depends on a number of factors: -The amount of combustible material that is exposed to high temperatures, as well as flames generated by burning cables; -the geometric shapes of cables and their mutual location when laying; -ignition temperatures of gases emitted by the cables; -the volume of combustible gases released by cables at a certain elevated temperature; -room capacity and the volume of air passing through the cable structure.
It was found that the spread of fire is affected not only by the volume of cables laid, but also by their mutual placement in space. As an example Figure 1 shows that five cables of general industrial version of the type VVG and NRG , in most cases, spread the combustion at the vertical location of samples. In this case, a steady spread of combustion is observed when the cables are located in a bundle with a gap, as shown in Figure 1. In this regard, all modern types of cables that do not spread combustion in the bunch (non-combustible version) are tested in the bundles with a normal volume of combustible materials (the number of cables) with or without a gap, depending on the types of cables and the characteristic method of laying them in cable structures. In accordance with [3], cables for nonproliferation of combustion in bundles are subdivided into 5 categories depending on the normalized mass volume of non-metallic structural elements. The most stringent standards are established for cables in test category A and AF/R, for which the normalized volume of combustible materials is 7 liters per 1 meter of cable bundle. Typically, in Russia, these requirements are set for cables for energy purposes (power, control and management). For other types of cables less stringent standards may be set, which are defined in the test categories of samples with a mass of combustible materials of 3.5 l/m (category B) or 1.5 l/m (category C).
In electrical installations where there are special fire safety requirements, it may be important to use special types of cables.
The use of cables that do not meet the requirements of standards to curb their ability to spread combustion, should be limited to small sections to connect devices to permanent wiring and in any case should not be allowed between rooms separated by fire walls.
Wiring components, except cables that do not meet the requirements of standards to curb their ability to spread combustion, but in all other aspects meet the requirements of standards, are to be placed in a shell of fireproof materials.
When installing FRCL a variety of mounts and fasteners are used. In the installation of fiberglass on the basis of trays and stair trays wall brackets are used.
In an open single and bunch wiring as part of the FRCL can be connected without using a cable tray -on single brackets and clamps that are attached to the walls.
Cable brackets are designed for laying and fixing cables and installing pipelines for industrial use indoors and outdoors.
Cable clamps are designed for laying single cables and cable bundles to the walls and floor.
Cable clips are designed for wall and ceiling paired and single wiring on flat surfaces. They allow you to complement the FRCL with cables of different cross sections.
Analysis of regulatory documents (GOST IEC 60331-21-2011 and GOST R 53316-2009), has revealed some key differences in methods and final results of tests. This comparative analysis is presented in Table 2.

Methods
Determination of the limit state of cables was carried out according to GOST IEC 60331-21-2011 "Testing of electrical and optical cables under the influence of flames. Preservation of operability. Part 21: Testing of electrical and optical cables under flame conditions. Testing performance and requirements to them. Cables for rated voltage up to 0.6/1.0 kV inclusive".
For tests power cable type VVGng(A)-FRLS was taken, which is fireproof. In addition to fire resistance, it has a reduced fire hazard and reduced smoke -and gas emission. It consists of copper wire conductor core, mica tape winding, polyvinyl chloride insulation with reduced fire hazard, inner and outer sheathing of PVC with low gas-smoke emission. Appearance of cable samples is shown in Figure 2: Determination of the serviceability of cable lines (FRCL) and the integrity of FRCL fasteners was conducted in accordance with GOST R 53316-2009 "Cable lines. Preservation of serviceability in fire conditions. Method of testing". Tests are carried out in a small-size furnace for testing building structures for fire resistance.
Cable laying was carried out on the furnace wall. At single wiring the cable is fastened with a screw with a ring with a pitch of 50 ± 5 cm.
Bunch wiring consisted of 8 groups of cables, for 3 groups cables were selected with the same section and a different number of cores, for 3 groups cables were selected with the same number of cores and a different cross section, 2 groups are a combination of cables with different cross sections and numbers of cores. Cable suspension system for group cabling consists of metal trays, fixed with perforated tape and studs.
Determination of fire resistance of the cable was carried out according to GOST IEC 60331-21-2011 "Testing of electrical and optical cables under the influence of flames. Maintaining operability. Testing of electrical and optical cables under the influence of flame.
Part 21. Testing performance and requirements to them. Cables for rated voltage up to 0.6/1.0 kV inclusive".

Fire Resistance Tests in accordance with IEC 60331-21-2011 State Standard
The results of fire tests on cable samples are given in Table 3.

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The appearance of the cable sample after test is shown in Figure 3:  The results of flame propagation tests on vertically arranged cable bundles are presented in Table 4. The appearance of cable samples after the test is shown in Figure 5: The installed cable lines on the wall of the test furnace and the scheme of installing cable lines are shown in Figure 6.  Life time of each cable line is presented in Table 5.  As a result of this test, all conductive cores of the cable samples were destroyed. It was found that the third and fourth groups of cable lines are the most effective.
The effectiveness of the third group is achieved by installing a special support tray, which protects the cable line from direct flame exposure, as well as installation of cable products such as "FRLS", which means that the insulation and cable sheathing is made of plastic that is flame-retardant and emits little harmful substances. In cable samples VVGnoncombustible(A) -FRLS 3*1,5 conductors burned at 565 and 717 seconds, and in cable samples VVGnon-combustible(A) -FRLS 3*10 at 1736 and 2004 seconds respectively. We can conclude that the thicker the core of the cable sample, the higher the time of performance of the cable product if we compare cables of the same brand, but with different cross-sections. However, mounted metal tray bent relative to its axis, which means that we need additional fasteners in the form of perforated tape in 3 places as mounted in the fourth group ( Figure  14.). In the fourth group of cable lines efficiency was achieved by installing a special support tray, which protects the cable line from direct flame exposure, as well as the by installation of structural fire protection (ceramic fiber) on a bundle of 7 cable samples. Analyzing the test table, we can see that the time of destruction of conductive cores also directly depends on the thickness of the cores themselves -1.5 mm 2 cores burnt up to 845 seconds, 4 mm 2 cores burnt up to 881 seconds , and 10mm 2 cores -up to 1377 seconds. The metal tray fixed in 3 places with perforated tape and studs has not changed its design position.

Conclusion
In this paper the parameters of fire resistance and performance of cable products under the influence of flame depending on the type of cables and the method of their installation were studied.
In the process of studying the theoretical and practical parts of laboratory research detailed methods of testing for each of the experiments were developed. The technique includes a description of the test equipment, the process of sample preparation, step-by-step testing, as well as evaluation of the results.
In the practical part of this study, experiments were conducted on each of the three methods of laboratory testing, which were determined in accordance with the objectives. Each test was to identify and determine the parameters of cable products performance, namely: -in the first laboratory test, satisfactory results were obtained, as cable samples VVGnoncombustible(A) -FRLS meet the requirements of the current GOST IEC 60331-21-2011: with a duration of fire exposure of 75 minutes, voltage was applied to the cable during the entire test.
-all cable products, which were tested in the 2nd experiment, successfully passed the laboratory tests, as the value of flame spread on the samples was less than 2.5 meters.
-In the third test, the most efficient cable lines were identified. Efficiency in this case was reached by means of variation of various kinds of cable products, the way of installation of fastenings and lining, as well as by possibility of constructive protection of cable products.
In the process of laboratory tests the direct dependence of the influence of the cable core cross-section on the preservation of the cable line operability under fire effect was revealed. That is, the thicker the core of the cable sample, the longer the operating time of the cable line under fire exposure.
Also, the third laboratory test showed more effective parameters for the preservation of operability under fire exposure in the case of bunch wiring, as opposed to single wiring, namely, the time of operability under fire exposure for the bunch is much higher than with a single cable.
Unfortunately, the results of tests for fire resistance of cable products in a single wiring according to GOST IEC 60331-21-2011 and in a bunch wiring according to GOST IEC 60332-3-22-2011 can not be extended to the results of tests according to GOST R 53316-2009 for the preservation of serviceability of cable lines, as the declared time of fire resistance of cable products -90 minutes does not apply to the time of fire resistance of the cable line. Often, the manufacturer guarantees the maintenance of the cable line during the time specified in the certificates for cable products. However, the documentation specifies the allowable time of fire exposure when assessing the fire resistance of the cable in a single or bunch wiring. Therefore, it should be understood that the assessment of fire resistance and performance of the cable and the cable line is carried out according to completely different testing methods.
Safety design of cable lines and subsequent installation is an integral part of integrated safety of construction of buildings and structures, as well as their further operation.
The results and conclusions of this research may serve as a basis for the design of cable lines and electrical installations, drafting technical documentation and specifications.
This work was financially supported by the Ministry of Science and Higher Education of the Russian Federation (Project: Theoretical and experimental design of new composite materials to ensure safety during the operation of buildings and structures under conditions.